1. Field of the Inventions
The present invention relates to an apparatus for recording data on an optical rewritable disk using a mark edge method and to an apparatus for recording reproducing the recorded data, and more particularly, to compensation on recording data in order to correctly read recording marks in the form of pulse trains and mark edge positions of a reproduced signal.
2. Description of the Prior Art
One of the advanced disk-shaped recording mediums capable of storing data at high density is known as a phase-change optical disk. Data can be recorded on the phase-change optical disk by irradiating a surface of an optical disk being rotated with a converged laser beam to heat up and melt the irradiated area of a recording layer. A difference of the power of the recording laser beam causes differences of the recording layer in arrival temperature and cooling process, and in turn causes a difference in the physical state or phase, of the recording layer. More specifically, an area irradiated with a high power laser beam becomes high in temperature and then cools down rapidly to become an amorphous phase. On the other hand, an area irradiated with a relatively low power laser beam becomes moderate in temperature and then cools down gradually to become a crystalline phase. The amorphous area is commonly called as a mark and the crystallized area is called as a space. In recording, binary data are is stored in a series of marks and spaces. It is also possible for the phase-change optical disk to erase old data and record new data simultaneously using a single laser beam, or to perform direct overwriting operations.
In reproduction, a lower power laser beam whose power is too low to induce a phase change irradiates the recording layer and its reflected beam is detected. Since the reflectivity is low on the amorphous mark and high on the crystallized space, a difference in the intensity of the reflected beam between the mark and the space is detected to obtain a reproduced signal.
Recording of data on phase-change optical disks is generally carried out by two known methods, a mark position recording method (MPR) and a mark edge recording method (MER). The MPR method records marks of a short, uniform length at different intervals so that the position of the marks correspond to the data. In the MER, method marks of different lengths are recorded at different intervals so that the start and termination edges of each mark correspond to the data. Accordingly, the MER method can generally record at a higher data density than the MPR method. There is generally an edge shift which is a difference of the position of an edge of the reproduced signal from its ideal position. As the edges correspond to the data in the MER, method the edge shift causes an increase in the error rate of the reproduced signal in the MER method. It is thus essential for realization of the high density recording in the MER method to accurately arrange the edges of each mark so as to be at their desired locations.
In the MER method marks having a greater length are recorded as compared with those of the MPR method. However, it is common on the phase-change optical disk for a rear half of each mark to become wide in width in a radial direction due to a heat storage effect of the recording layer when a long mark has been recorded by irradiation of a uniform laser beam. This event will lead to incomplete erasing in direct overwriting or crosstalk between tracks during reproduction, impairing the recording/reproducing characteristics. For preventing the width of the mark from becoming wider in the radial direction in the rear half of the mark, techniques have been introduced in which the power of the laser beam is lessened at the rear half of each mark by controlling the power of the laser beam or the recording pulse width so that the width of the mark is uniform (for example, see Japanese Laid-open Patent Publication Nos. 5-151638 and 3-185628).
There is a substantial disadvantage of those techniques. As described, the marks on a phase-change optical disk are lower in reflectivity of light than the spaces whereby a difference in absorption of the light is created between the marks and the spaces. Also, heat for melting the amorphous phase portion is different from the heat for melting the crystalline phase portion. In direct overwriting, when new data is recorded on an existing mark or space with an equal intensity of laser beam, edges a new mark are changed in location because of differences in absorbed energy and the arrival temperature. Particularly, when the irradiation of the laser is lowered at the rear half of the mark for the improvement in the shape of the mark during recording, the amorphous phase of the rear half of the mark tends to be unstable, thus producing the edge shift of a termination end of the mark during the direct overwriting.
In addition, three more disadvantages are developed when the marks and the spaces are minimized in size for a high density recording. Firstly, a shorter length of the space produces thermal interference in that heat at the termination end crosses the space to increase temperature of a start end of a succeeding mark and heat at the start end of the succeeding mark affects the cooling process of the termination end of the mark. Such thermal interference causes a change in the location of the edges of the mark. Secondly, since the mark with a shorter length is produced by heating a smaller region of the recording layer as compared with the mark with a longer length, a length of the mark tends to be unproportional to a length of a corresponding signal data to be recorded because of the change of condition of the heat radiation. Hence, the recording conditions based on the marks with the longer length can hardly be implemented with the marks with the shorter length. Thirdly, it is also common that even if the marks and the spaces are recorded at their correct locations on the phase-change optical disk, the positions of the edges of the marks or the spaces with the shorter length are incorrectly reproduced because the frequency response characteristics decrease at a high frequency in a reproducing optical system. The frequency characteristics during reproducing may be equalized so as not to cause the edge shift. However, this does not conform to a favorable requirement that the reproduced signal have an improved S/N (signal-to-noise) ratio so as to have less noise. In other words, the edge shift will increase as the S/N ratio is enhanced.